Obscure federal rules of civil procedure

In the book that I'm scrambling to finish (yes, I'm cite-checking on Christmas Day), I'm writing about a federal libel trial, litigated in 1927 in Detroit's district court. In this trial the plaintiff in the suit called a number of witnesses who were employees of the defendant, Henry Ford, and his newspaper. Naturally, these witnesses were hostile to the plaintiff, and the plaintiff's attorney approached them as such. In fact, what the plaintiff's lawyer wanted to do was to impeach their testimony.

From what I can divine from the trial transcript and the newspaper coverage, the only way the plaintiff's attorney could proceed to impeach his own witnesses was if defense counsel cooperated and agreed to cross examine the opposition's witnesses. The defense lawyers easily defeated this strategy by declining to cross examine their client's employees. What this meant, the press speculated, was that the plaintiff could not subject Henry Ford to rough treatment on the stand when Ford was called to testify as a plaintiff's witness. Ford had, in fact, been served with a subpoena, although his representatives denied for months that he had received the document.

My question is, where would I go to find a source for the federal rule of civil procedure that governed this situation? All I have is the remarks of counsel and the commentary by the press. Is that sufficient? I admit that I can't imagine the field of FRCP history as being terribly populated, but I'm happy to be surprised.